1. Field of the Invention
This invention relates to an electric motor having a permanent magnet in a rotor, such as Brushless DC motor or the like and, more specifically, to an electric motor in which the magnetic flux density, a reluctance torque and so on can be selectively established, appropriate for a compressor of an air conditioner for example.
2. Description of the Related Art
In the electric motor of the type as described above, a permanent magnet is embedded in a core of an inner rotor of the electric motor, an example thereof being shown in FIG. 26 which is a plan view showing the inside of this electric motor from a plane orthogonal to the axis of rotation.
In the drawing a rotor core 2 is disposed inside a stator core 1, for example, having 24 slots, in which a field magnet rotates. In this case, the number of poles of the electric motor is four, therefore four permanent magnets 3 are arranged in the rotor core 2 in accordance with the number of poles.
The permanent magnets 3 are each formed to be a band plate shape of rectangular cross-section, and are arranged to have equal spaces on the outer circle side of the rotor core 2 in the circumferential direction, to be embedded inside the rotor core 2 along a direction perpendicular to paper of FIG. 26.
Between the adjacent permanent magnets 3, holes 4 as a flux barrier for avoiding short-circuiting and leaking of the magnetic flux in between the adjacent permanent magnets 3, are formed. In this example, the hole 4 is represented as a triangle hole and disposed at each end of the permanent magnet 3. In the center of the rotor core 2, a center hole 5 is formed to pass a rotating shaft (not shown) therethrough.
In this point, when the magnetic flux distribution in a gap portion (between teeth of the stator core 1 and the permanent magnets 3) caused by each permanent magnet 3 is in a sine wave state, torque T of the electric motor is given as T=Pn{.PHI.a.multidot.Ia.multidot.cos .beta.-0.5(Ld-Lq).multidot.I2.multidot.sin 2.beta.}, where T is an output torque, .PHI.a is an armature flux-linkage caused by the permanent magnet 3 on the d and q coordinate axes, Ld and Lq are the d-axis inductance and the q-axis inductance respectively, Ia is amplitude of an armature current on the d and q coordinate axes, .beta. is a lead angle of the armature current from the q axis on the d and q coordinate axes, and Pn is a pole-logarithm.
In the above expression, the first term expresses a magnet torque generated by the permanent magnets 3 and the second term express a reluctance torque generated by the difference between the d-axis inductance and the q-axis inductance. Refer to a treatise published in T. IEE Japan, vol. 117-D, No. 8. 1997 for further detail.
Typically, a ferrite magnet and a rare-earth magnet are used for the permanent magnet 3 employed in the aforementioned type electric motor.
The ferrite magnet is low cost and available for forming the permanent magnets in various configurations due to it's ease of shaping, but the magnet flux density is low, therefore hindering the reduction in size of the rotor core.
On the other hand, the rare-earth magnet has a high magnet-flux density, so that the reduction in size of the rotor core can be easy, but the configuration of the permanent magnet is limited by the difficulties of shaping thereof. In addition, the rare-earth magnet has a higher cost than the ferrite magnet.
Since both the ferrite magnet and rare-earth magnet have the pros and cons as explained above, conventionally for reasons of the use of a motor and/or a cost, either the ferrite magnet or the rare-earth magnet is chosen for all permanent magnets of magnetic poles.
In addition to the cost aspect, since all permanent magnets forming the magnet poles have the same shape as shown in FIG. 26, a range for determining the magnetic flux density, the reluctance torque and the like is narrow, thereby causing problems in designing the electric motor. As to the configuration of the permanent magnet, for example, an inverted arc shaped permanent magnet is referred to in the aforementioned treatise, but still in this case, all permanent magnets used for all poles have the same shapes.
For example, when all magnetic poles are formed of the same rare-earth magnet, the magnetic flux density is excessively high and also the cos higher. When all magnetic poles are formed of the same ferrite magnet, despite the low costs, the magnetic flux density is insufficient, resulting in not obtaining of a sufficient motor torque.
The shapes of the permanent magnets of all magnetic poles are the same, whereby the reluctance torque is determined on one ground.
As described hereinbefore, conventionally, proper permanent magnets having an intermediate state between the ferrite magnets and the rare-earth magnets are troublesome to obtain, that is to say it is difficult to select the required magnetic flux density, reluctance torque and cost.